Telephone having a speech ban limiting function

Details Telephone having a speech ban limiting function Telephone having a speech ban limiting function

1997-09-16

2001-02-06

Chan, Wing F. (Department: 2743)

Telephonic communications

Substation or terminal circuitry

For loudspeaking terminal

C379S387020, C381S094100

Reexamination Certificate

active

06185298

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a telephone capable of limiting a band-width of a received speech signal and, more particularly, to a mobile telephone capable of making a received speech easy to hear even in a noisy environment.
It has been customary with a telephone to include a filter for limiting a band-width of a received speech signal in a receiver circuit thereof. The filter limits a received speech signal to a predetermined frequency band and thereby makes the speech easy to hear. Specifically, the telephone has a casing and a transmitter (microphone) and a receiver (speaker) mounted on the casing. A radio section demodulates a signal coming in through an antenna. The filter mentioned above controls the frequency band of the output of the radio section. The radio section and filter are connected to the receiver. In this configuration, the demodulated signal from the radio section has the frequency band thereof limited by the filter, so that a speech signal is output from the receiver within the limited frequency band. Usually, the filter is of the kind filtering out a low frequency component in order to make a received speech easy to hear.
Various approaches have recently been proposed to improve the quality of speeches received on a telephone. One of them is to provide the filter with a characteristic which enhances a low frequency component by filtering out a high frequency component. Specifically, when a high frequency component is enhanced to promote easy hearing, a received speech sounds metallic and annoys the user of the telephone. In contrast, the filter of the kind filtering out a high frequency component allows a received speech to sound soft, thereby improving the speech quality. Another approach is to allow the user to adjust the frequency of a received speech on the telephone, i.e., the easiness of hearing, as taught in Japanese Patent Laid-Open Publication No. 1-123554 (document 1) by way of example.
However, the telephone with any of the conventional implementations for improving speech quality has the following problem. Assume that the telephone is used in a noisy urban environment outside of a building. Then, because city noises are generally distributed in a low frequency component, they are superposed on a speech signal whose frequency is limited to a low frequency component. The resulting speech output from the receiver is extremely hard to hear. For this reason, it has been customary to limit received speeches to a high frequency band so as to promote easy hearing, rather than to improve the speech quality in consideration of operation in noisy environments.
The telephone taught in the above document
1
promotes easy hearing because it allows the frequency characteristic of a received speech to be adjusted in matching relation to the environment. However, the adjustment is done by hand and, therefore, forces the user to manipulate the telephone while conversing with the other party. Moreover, it is likely that noise drowns out a speech due to inadequate adjustment and shuts off communication for a moment.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a mobile telephone capable of improving the quality of a received speech and, in addition, promoting easy hearing of the speech even in a noisy urban environment.
It is another object of the present invention to provide a mobile telephone capable of improving the quality of a received speech and promoting easy hearing in a noisy environment by automatically changing the frequency band of the speech in matching relation to the noise level.
It is a further object of the present invention to provide a mobile telephone which is adjustable to an easy-to-hear condition at all times without resorting to manual operation.
A telephone of the present invention has a speech band switching circuit for selectively providing a received speech with one of a plurality of frequency characteristics. A sensing circuit senses a noise level. A speech band control circuit compares the noise level sensed by the sensing circuit with a predetermined level, and controls the speech band switching circuit on the basis of the result of comparison.
The speech band switching circuit may comprise a plurality of band-pass filters each for providing a received speech with a particular frequency characteristic, and selectors for selecting one of the band-pass filters at a time. The sensing circuit may comprise a microphone mounted on the casing of the telephone for sensing noise around the telephone.
The telephone having the above construction can automatically select the high frequency component or the low frequency component of a received speech on the basis of the surrounding noise level. When the noise level is high, the telephone ensures communication by promoting ease of hearing while sacrificing speech quality. For example, in a high noise level environment, the telephone selects the high-pass filter to filter out a low frequency component, thereby rendering a received speech easy to hear. In a low noise level environment, the telephone selects the low-pass filter so as to filter out a high frequency component and thereby causes a received speech to sound soft, i.e., improves speech quality. Further, the telephone of the present invention determines whether or not sound input to a transmitter is voice or noise on the basis of the level of the sound, and selectively enables or disenables the control circuit. The transmitter, therefore, plays the role of a noise detector at the same time.


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